1. Field of the invention
The present invention relates to measurement of a crystal face orientation, and more particularly to a method and an apparatus of measuring the crystal face orientation of a crystal material.
2. Description of the Prior Art
A semiconductor crystal of, for example, Group IV semiconductor such as silicon (Si) or compound semiconductor such as gallium arsenide (GaAs) and gallium phosphide (GaP), is cut out in the form of a wafer as a semiconductor device substrate which is commercially available. There has been known an X-ray diffraction method which obtains the information of interplanar spacing of a bulk crystal to measure the surface orientation.
According to another method described in Japanese Patent Unexamined Publication No. 58-210546, a crystal orientation is obtained from the rotational angle dependency of the intensity of Raman light that emerges from the crystal face.
The first conventional method above mentioned, however, has the disadvantage that the cost of measurement of crystal face orientation becomes increased, because it requires a large, expensive X-ray apparatus and also requires a safety countermeasure for X rays. The second method using the rotational angle dependency of the intensity of Raman light also has the disadvantage that high-precise measurements cannot tee achieved because of the week signal intensity of Raman light. In order to overcome the aforementioned disadvantages, we would like to propose a novel measuring method and apparatus.
An object of the present invention is to provide a method and apparatus which can precisely measure a crystal face orientation with reduced amount of hardware and cost.
Another object of the present invention is to provide a method and apparatus which can improve the signal-to-noise ratio to achieve the precise measurement of a crystal face orientation.
First, a description will be made of a method for obtaining a crystal face orientation from the angle dependency of the intensity of the second harmonic. When the surface of the crystal is irradiated with light of a certain wavelength, non-linear polarization will be induced and thereby light with a wavelength of half that of the incident light, called a second harmonic, will be emitted. The magnitude of polarization varies according to the direction of atom bonding within the crystal. Therefore, if linearly polarized light is employed as incident light, the intensity of emitted light will change in accordance with the arrangement of atoms within the crystal. Thus, it is possible to know a crystal face orientation by monitoring a change in the intensity of the light emitted from the surface of the crystal.
According to the present invention, a crystal face orientation is measured based on the angle dependency of the intensity of reflected light. The wavelength dependency of the reflectance coefficient of a crystal in the ultraviolet range exhibits a peak which reflects the energy band structure of the crystal. Since the energy band structure depends upon a crystal axis, the reflectance coefficient of incident light near the peak depends upon a relative angle between the direction of polarization of incident light and a crystal axis. Therefore, by irradiating a crystal face with light having a photon energy which corresponds to the specific transition energy of the crystal and also detecting the rotational angle dependency of the reflectance coefficient within the crystal plane, the crystal face orientation can be obtained.
However, since the reflected light from a crystal is very weak, it is very difficult to sufficiently measure a change in the intensify of the reflected light as it is. When a crystal is in room temperature, the lattice momentum is small. When the lattice momentum is near zero, the difference of the bandgap with respect to the crystallographic axis is small and therefore the difference of the reflectance coefficient with respect to the crystallographic axis is also small. For this reason, even if the reflectance coefficient were rotated within a crystal face while detecting the rotational dependency, the dependency would be too small to perform high precision measurements.
Therefore, we employ a means of increasing the signal-to-noise ratio. According to a first aspect of the present invention, a sample is rotated about an axis perpendicular to a surface of the sample in predetermined angular steps and a crystal lattice of the sample is excited or stimulated. Irradiating the surface of the sample with linearly polarized light, a reflected intensity of light reflected from the surface of the sample is detected in each angular step. Based on a rotational angle dependency of the reflected intensity, the crystalface orientation of the sample is determined.
Exciting the lattice momentum of the sample increases a change of the reflectance coefficient with respect to the crystallographic axis. For example, by heating or irradiating the sample with light or ultrasonic waves, it becomes possible to render the lattice momentum larger, and a crystal face orientation can be precisely measured.
We employ another means of increasing the signal-to-noise ratio. According to a second aspect of the present invention, the surface of the sample is irradiated with a plurality of linearly polarized light beams in each angular step. A plurality of positions on the surface of the sample may be irradiated with the linearly polarized light beams, respectively, the linearly polarized light beams being generated from a single linearly polarized light beam. The linearly polarized light beams may be formed by irradiating the surface of the sample with a single linearly polarized light beam a plurality of times in the same direction in each angular step. The means of exciting the crystal lattice of the sample may be combined to further improve the signal-to-noise ratio.
We employ still another means of increasing the signal-to-noise ratio. According to a third aspect of the present invention, a first linearly polarized light beam and a second linearly polarized light beam are generated from a single linearly polarized light beam. The surface of the sample is irradiated with the first linearly polarized light beam. A reflected intensity of light reflected from the surface of the sample and an intensity of the second linearly polarized light beam are detected in each angular step. The crystal face orientation of the sample is determined based on a rotational angle dependency of the reflected intensity adjusted by the intensity of the second linearly polarized light beam. The means of exciting the crystal lattice of the sample may be combined to further improve the signal-to-noise ratio.
We employ further still another means of increasing the signal-to-noise ratio. According to a fourth aspect of the present invention, the surface of the sample is irradiated with a first linearly polarized light beam and a second linearly polarized light beam in first and second directions, respectively, the first and second linearly polarized light beams having the same wavelength and the same direction of polarization. A first reflected intensity of light reflected in the first direction from the surface of the sample is detected in each angular step, and a second reflected intensity of light reflected in the second direction from the surface of the sample is also detected in each angular step. The crystal face orientation of the sample is determined based on a rotational angle dependency obtained from the first and second reflected intensities. The means of exciting the crystal lattice of the sample may be combined to further improve the signal-to-noise ratio.
According to the present invention, as previously described, it becomes possible to measure a crystal orientation with an improved signal-to-noise ratio and a high degree of accuracy using a structurally simple apparatus.